This invention relates to compact imaging systems, and, more particularly, to imaging systems having small telephoto ratios.
An imaging lens (or lens system, also referred to as an imager) is commonly used to observe objects or scenes, whereby light emitted or reflected by a given object or scene is imaged by some means onto a detecting element, where it can readily be observed or recorded.
Because the field of view of an imaging lens is related to its focal length, narrow field of view imagers tend to have longer focal lengths. It is therefore desirable in many cases to constrain the overall length of the optical system while maintaining a long focal length. The telephoto ratio of an imaging system is given as the ratio of the overall length, measured from the foremost optical element to the image plane (or detecting element), to its effective focal length. This is achieved in a number of ways including the classical solution comprised of positive and negative powered element groupings as well as folded mirror or catadioptric systems. While these approaches can provide some shortening of long focal length imaging systems, they have practical limitations.
An afocal lens system is a system that images an object, located a large distance away from the system, to an image plane, also located a large distance away from the system, with some angular magnification. Afocal attachments have conventionally been used to change the effective focal length of an optical imaging system by providing angular magnification at the pupil.
In many designs incorporating optical systems, there is a need for the overall system to be compact. Such needs stem from weight and space constraints in the application in which the system is used. Conventional afocal attachments have been used to change the focal length of an optical imaging system, but are limited in their ability to provide small telephoto ratios, since they contribute substantial size and mass to the imaging system in order to achieve the desired angular magnification and field of view.
There is therefore a need for an imaging system that is more compact in physical size than current imaging systems.
Furthermore, there is also a need for an imaging system that has a smaller telephoto ratio than current imaging systems.
Still further, there is a need for an imaging system that provides a combination of the characteristics described above with superior trade-offs than have been previously attainable.